While the prior art has contemplated various and diverse time-of-flight systems for discerning the location of a radio transmission source, basic systems have tended to provide somewhat limited, two dimensional location data, neglecting the Z-Axis, which corresponds to height, or the third dimensional location of the transmitter. Such prior art systems have been adequate where location of the transmitter was fixed in a relatively flat, planar location field, but would be inadequate for utilization where precise, three dimensional location was a necessity.
A prior listing of patents believed to have at least some cursory pertinence to the present invention is provided below:
______________________________________ Patent Number Inventor(s) Date of Issue ______________________________________ 5,225,842 Sturza et al 07/06/1993 5,210,540 Masumoto 05/11/1993 4,731,613 Motoki et al 03/15/1988 ______________________________________
Typically in a radio location system, receiving towers are placed at distant locations. Due to the economics and cost of each receiver station, it is desirable to locate these receivers as widely separated as possible. This has the effect of placing the receivers in approximately the same plane of operation of one another.
GPS systems, utilizing multiple satellites to fix location, have the advantage of considerably increased altitude, which improves the intersection geometry of the lines of position. GPS systems, however, have difficulties at fixing positions at very great travel distances. These distances increase the overall path loss and require reasonably good antenna view and antenna gain. In a system intended to operate from within a multi-story building, the signal attenuation of a GPS system would tend be too great to provide an adequate signal margin to compute a position fix.
In ground based receiving systems, the solution is to place additional receivers at increased heights, in order to improve geometries of intersection, for a three dimensional fix. This has the disadvantage of additional cost. This has the further disadvantage of propagation path constraints forced by the building's construction.
Altitude sensors have been used in combination with GPS receivers in order to reduce the number of receivers which must be in view in order to provide a three dimensional position fix, or even a two dimensional position fix; see, for example, U.S. Pat. No. 5,210,540. See also U.S. Pat. Nos. 5,225,842, 5,265,025 or 4,731,613. The disclosures in the prior art teach techniques whereby a receiver may better estimate its own position, so long as that position is in reasonably clear view of one or more GPS satellite transmitters.
In summary, the known prior art does not disclose techniques which would locate an unknown positioned transmitter emitting a signal greatly attenuated because it is located deep within a multi-story building. Further, the prior art references do not use transmit only techniques which allow for the increased battery life and small size required to make this application practical.
Lastly, the above references do not teach utilizing a mobile reference unit to increase the accuracy and enhance the ability to find an unknown positioned transmitter within a building.